I had to get some help from Hollywood on this one. My brother'* girlfriend used to do graphics work for some movie studios in CA. I asked her to call up some friends in the special effects department to see if they could give me some help. They led me to exactly what I was looking for. Liquid foam...
It comes as a two-part kit that is mixed 1:1 by volume. I also bought a release agent that didn't seem to work very well. Here is parts A & B ready to be mixed along with the plaster mold:
Here it is after 10 minutes in the mold. The foam is self-skinning and expands 6 times in volume.
Here is a cross section of the unused portion that was removed from the cup. The density of the foam is rated at 10lbs/sq foot although it does seem to be denser than that:
The release agent sucked. It was more difficult to remove the foam from the plaster mold (which received a coating) than it was to remove from the cup. Some of the detail and sharp edges stuck to the mold but overall a sucess for the first run:
I've already sent the door panels to be embroidered because I wasn't completely happy with the results. Since I think the foam will work well for this application, I'm going to continue to refine the process.